Headsets are often used where the need arises for "hands free" telephone operation. In order to enable headset operation with existing, conventional telephones, an interface/amplifier apparatus is required. Such apparatus connects together the telephone, its headset and the handset and permits headset or handset selection with a transfer switch. DC power is needed for headset interface/amplifier apparatus operation.
There are various telephone instrument designs in existence. In terms of headset interface requirements they fall under one of four categories, based on handset type: carbon, dynamic, electret and DC powered "three wire". Carbon handsets, when off hook, pass typically 20-60 mA of direct current through the carbon microphone. Such current is well in excess of the few milliamperes needed for a headset amplifier, and such carbon telephone headset amplifiers are available today, for example the carbon telephone headset amplifier GSA 50 from VXI Corporation of Rollinsford, N.H.
Dynamic microphone handsets receive no power from the telephone instrument and are a clear example of why a headset amplifier intended for such telephones requires DC power. Electret mic handsets operate at approximately 200 .mu.A and low voltage. Again, a headset amplifier requires a separate DC power source because the current obtainable from the telephone is limited by a high value resistor, typically 2.2 k.OMEGA.. DC powered "three wire" handsets, for instance R type handsets used with AT&T Merlin.RTM. telephones actually have four connections: +DC power, transmit, receive and ground. The original intended purpose of the DC power is to operate a transmit and receive amplifier embedded in the handset.
Universal telephone headset amplifiers have been available that accommodate the handset variations. They rely on batteries or AC power adaptors. These devices may have as many as 18 DIP switches that the user is required to configure for the respective telephone by referring to a chart or calling the manufacturer. Hello Direct model Pro Amplifier.TM. is an example of such a product. It will be appreciated that with 2.sup.18 =262,144 DIP switch possibilities, great care and accurate information is required for proper amplifier configuration.
A solution that avoids such switches and operates without batteries is described in U.S. Pat. Nos. 5,623,544 and 5,544,243 and has been sold as model GSA 60 by VXI Corporation. This interface/amplifier circuit is only suitable for so called electronic telephones, i.e., instruments with electret or dynamic handsets. Since this product does not operate with carbon or R type handsets, it is inappropriate for a large share of the telephone headset market. Call centers are often equipped with carbon or AT&T Merlin.RTM. (and other R derivative) telephone stations.
The voice signal levels provided at the telephone connector for handset receive are comparable among the aforementioned four telephone handset types, typically 10 mv. Headset amplifiers operate with this input level and provide gain, a volume control function and acoustic limiting (also referred to as AGC) when driving the headset receiver or receivers.
The transmit signal presented at the telephone connector by the handset microphone may be at one of two levels: tens of millivolts with dynamic or electret handsets and hundreds of millivolts with carbon or R type handsets. The transmit impedance is similarly at one of two levels: hundreds of ohms for dynamic or electret and tens of ohms for carbon or R type.